Method for producing a paper web, and paper machine

ABSTRACT

The present invention relates to a method and a paper machine for producing a calendered paper web. The method includes the step of partially loading with ash a plurality of fibers of a fibrous stock suspension. The paper machine for the production of a calendered paper web from a fibrous stock suspension including a plurality of fibers which are partially loaded with ash includes a first device for loading a fibrous stock suspension with ash. The first device can include a first static mixer, a preparation unit for adding calcium oxide or calcium hydroxide, a press or a dewatering screw, an equalizing reactor or an equalizing screw, a container serving as a crystallizer, a second static mixer, and a carbon dioxide storage tank or a second device for recovering carbon dioxide.

SUMMARY OF THE INVENTION

The present invention provides that the paper web is produced by using afibrous stock suspension which contains fibers that are partially loadedwith ash.

When supplying ash with the assistance of the Fiber-Loading-Process, forexample according to DE 102 04 254A1 the ash is precipitated directly inthe fibers and adhered to them. As detectable through microscoperecordings, ash is also deposited in the cavities of the fiber.Blackening is avoided since even with collapsed fibers a fiber-ashinterface and an ash-fiber interface exist.

This means that according to the current invention the blackening index,however, at the same final roughness is reduced by at least 10%. Viceversa, the final roughness in the paper is reduced by at least 12% atthe same blackening index.

One particularly advantageous embodiment provides that the paper web isrun through a calender in the paper machine which is equipped with amultitude of nips wherein line loads greater than 50 kN/m are applied tothe paper web in the nips.

It is especially advantageous if line loads greater than 100 or greaterthan 350 kN/m are applied to the web.

It is also advantageous if the paper web is dried to a moisture contentof less than 5%, and is subsequently remoistened prior to the calenderto a moisture content of more than 7%.

The invention also relates to a paper machine for the production ofsupercalendered paper according to one of the aforementionedembodiments.

The paper machine is characterized in that it includes a device forloading a fibrous stock suspension with ash. The advantages of saiddevice apply to online as well as to an offline production of the paperweb, in other words, even if the device for loading the fibrous stocksuspension and the machine for the production of the paper web are notarranged in tandem and the paper manufacturing process occursimmediately following the loading of the fibrous stock suspension withash. Due to the fiber loading the paper can endure higher line loads inthe calender and a greater smoothness of the paper web can be achieved.

It is advantageous if the device includes a static mixer, a processingunit for the addition of calcium oxide or calcium hydroxide, a press ordewatering screw, an equalizing reactor or an equalizing screw, acontainer serving as crystallizer, an additional static mixer, a carbondioxide storage tank or an additional device for the recovery of carbondioxide.

One advantageous embodiment provides for a high consistency cleanerand/or a carbon dioxide heater and/or a storage tank for press water orfor water which was removed in the dewatering screw.

It is also advantageous if filtrate from the fiber stock suspensionwhich is recovered in the dewatering screw is returned through a line toa header tank or to another upstream device for processing of thefibrous stock suspension.

Another advantageous provision would be an additional washer unit forcleansing of the fibrous stock suspension following the container thatis serving as crystallizer.

In addition it is also advantageous if the paper machine also includes atwin wire former. The press section which is located downstream fromsaid twin wire former would include at least one shoe press. The papermachine includes one calender with a multitude of nips in order toreduce roughness and increase smoothness. An arrangement forremoistening of the paper web is located before the calender. Theremoistening is advantageously accomplished by way of a steam blow boxwhich also increases the gloss of the paper.

In the production of LWC and MWC papers (LWC=light weight coated;MWC=medium weight coated), a coating unit is of additional advantage.

A completely new product possessing new and improved characteristicscompared to products which are already on the market will result if afibrous stock suspension is treated with the fiber loading technologyduring the paper manufacturing process. The process described belowpermits precipitation of a filler (calcium carbonate) which is evenlydistributed and deposited only on and in the fibrous stock—especiallythe paper fiber—to occur directly in the stock preparation in the papermill.

Due to the fact that only fibrous stock loaded with precipitated calciumcarbonate is produced, wherein the calcium carbonate is attached on orin the fibers, or is embedded in them the formation of looseprecipitated calcium carbonate (PCC) is prevented. An additional washcycle before a refining process and/or after the refining process and/orbefore the crystallization process in a crystallizer and/or before aheadbox chest or prior to delivery into the paper machine or recyclingthe press filtrate to a header tank or to another storage arrangementlocated on the infeed side provides that a constant calcium hydroxidecontent is adjusted or regulated in the infeed system of the fiberloading device. The calcium hydroxide can be added directly in a fiberstock pulper. The press filtrate can be recycled into the pulper system.Calcium hydroxide which does not attach on or into the fibers isrecycled back to the upstream process.

Only that filler which is not attached on or in the fibers, in otherwords loose precipitated calcium carbonate, is washed out. The fibersthemselves which are provided with filler on the inside and the outsidedo not lose said filler through the wash process and the recycling ofthe press filtrate, so that the positive effects of the fiber loadingtechnology are maintained.

The present invention especially also provides that the fibrous stocksuspension is fed into a press arrangement intended to squeeze out afiltrate. Subsequently, the filtrate is recycled back, at leastpartially into an arrangement for pulping of the fibrous stocksuspension, specifically into a reservoir which is located on the infeedside, for example a header tank. The calcium hydroxide is added at leastpartially in the arrangement for pulping of the fiber stock. In thecomplete pulper system, specifically in the arrangement for pulping ofthe fibrous stock, a pH value of between 7 and 12, especially between 9and 12 is maintained.

Calcium hydroxide in aqueous or in dry form, or calcium oxide are mixedinto the aqueous paper fiber stock in a range of between 0.01 and 60% ofthe existing solids content. A static mixer, a header tank or a pulpersystem are utilized for the mixing process; a pH value in the range ofbetween 7 and 12, preferably between 9 and 12 is applied. The reactivityof the calcium hydroxide is between 0.01 seconds and 10 minutes,preferably between 1 second and 3 minutes. Dilution water is addedaccording to predetermined parameters.

Carbon dioxide is added into the moist paper stock suspension accordingto the reaction parameters. In doing so calcium carbonate precipitatesin the carbon dioxide atmosphere.

At the same time a refining energy in the range between 0.1 and 300kWh/ton dry paper pulp is applied. Compared to conventional processesfor the production of a fibrous stock suspension, the current inventionprovides energy efficient attainment of a higher level of freeness;according to the current invention as much as 50% of refining energy canbe saved.

The high mechanical strengths in the end product which are achievedthrough the high freeness value positively affect the production of allpaper grades since, due to process based mechanical loads in the varioussections of the paper machine, such as in the press section, the dryersection or in the section where the web is wound, the producedintermediate product and the end product which is to be produced bear ahigh mechanical load due to utilization of winders, rewinders andconverting machinery. The energy supply during the refining process,specifically the heat volume and the resulting warming effect arecontrolled. Crystals in various forms can be produced, according to thecontrol.

The pre-treatment of the fibrous stock suspension also creates the basisfor improved drying, thereby increasing the efficiency level in theproduction of all paper grades. Residual moisture contents in the rangeof 1 to 20% are advantageous.

With the current invention greater brightness and/or higher opticalvalues with an improved brightness of up to 15 or more lightness pointsare also achieved on all grades of paper.

It may also be provided that an expenditure of energy of between 0.3 and8 kWh/t, especially between 0.5 and 4 kWh/t is used for theprecipitation reaction, especially if no refining process is utilized.

The process temperature is preferably between −15° C. and 120° C.,especially between 20 and 90° C. Rhombohedral, scalenohedron andspherical crystals are formed, wherein the crystals measure between 0.05and 5 μm, especially between 0.3 and 2.5 μm.

Static and/or moving, especially rotating, mixing elements are utilizedfor the production of a fibrous stock suspension which is loaded withcalcium carbonate.

The process may be conducted in a pressure range of between 0 and 15bar, especially between 0 and 6 bar. Also, the process is carried out ata pH value that is preferably between 6 and 10, especially between 6.5and 8.5. The reaction time is between 0.01 seconds and 1 minute,especially between 0.05 seconds and 10 seconds.

An additional advantage when utilizing the present invention with theabove referenced paper grades is that these can also be processedfurther in a calender. Blackening is avoided due to the fact that whenusing the fiber loading technology, fiber loading particles aredeposited in, around and on the fibers.

Compared to a fibrous stock which is produced according to conventionalmethods a fibrous stock produced in accordance with the fiber loadingcombination process technology possesses a superior dewatering capacitywhich is in the range of between 5 to 100 ml CSF or 0.2 to 15° SR,depending upon the required level of freeness and filler content. Thisfibrous stock possesses a lower water retention capacity ofapproximately 2 to 25%, depending upon the raw material that is used inthe paper manufacturing process. Compared to conventional fibrous stockthe water can be removed more quickly from the fibrous stock suspension,and the fibrous stock dries accordingly faster. This also has a positiveeffect on remoistening which, as a result, is diminished in the papermanufacturing process, and upon the printability of the produced papergrades.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic drawing illustrating the preparation of a fibrousstock suspension for application in a machine for the production of afiber web;

FIG. 2 shows blackening as a function of the roughness in a paper web;and

FIG. 3 shows a paper machine for the production of calendered paperaccording to the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate an embodiment of the invention, and such exemplifications arenot to be construed as limiting the scope of the invention in anymanner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown a pipe line system 1 that is equipped with control valves 2, 3which is provided for a fibrous stock suspension. The control valve 2 islocated in a line 4 through which the pipe line system 1 is connectedwith a static mixer 5. Dilution water is added to the mixer 5 via avalve 6. A tank 7 or a container for storage of the fibrous stocksuspension is located following the mixer 5, viewed in a direction ofthe fiber stock flow direction. From the tank 7 the fibrous stocksuspension is pumped via a pump 8 to an additional static mixer 9.Dilution water is also added to the mixer 9 via a valve 10. Likewise,the inflow of a calcium hydroxide suspension is controlled through avalve 11 which is located in a line 12.

This is supplied by a processing unit 13 where solid calcium oxide orcalcium hydroxide is added to water. For this purpose the processingunit 13 is supplied with water via a line 14 which is equipped with avalve 15. The suspension that is produced in the processing unit 13 issupplied through a pump 16 into the line 12.

The fibrous stock suspension to which calcium hydroxide was added flowsfrom the mixer 9 into a line 17 which is equipped with a valve 18 to adewatering screw 19 where water is removed from the fibrous stocksuspension. Said water can, for example, be returned through a line 20to the mixer 5 as dilution water. Alternatively, or in addition thewater that was removed in the dewatering screw 19 may also be routed toa storage tank 21 for the fibrous stock suspension, or it is returned tothe mixer 9. Due to the return flow of calcium hydroxide containingwater the pH value can be increased and adjusted in all instances in theunits that are located preceding the dewatering screw 19.

In order to equalize the fibrous stock suspension, said suspension isdelivered via a line 22 from the dewatering screw 19 to an equalizingscrew 23. A downstream container 25 (crystallizer) is connected througha line 24 with said equalizing screw. For the purpose of supplyingcarbon dioxide this container is connected with a carbon dioxide storagecontainer 30 via a line 29 which is equipped with valves 26, 27 and apump 28. Carbon dioxide is supplied from this container into thecrystallizer 25 in order to produce the desired precipitation reactionof calcium hydroxide and carbon dioxide for the formation of calciumcarbonate as a filler in the fibers of the fiber stock.

In addition the carbon dioxide storage container 30 is connected withthe equalizing screw 23 via an additional line 31 which is equipped witha valve 32 and which branches off line 29. Carbon dioxide can herewithalso be supplied to the equalizing screw in order to achieve at least apartial precipitation already at this point.

Line 29 is also connected via an additional valve 33 with a static mixer34. This serves to add additional carbon dioxide to the fibrous stocksuspension which is flowing from the crystallizer 25 via a line 36 whichis equipped with a valve 35.

The fibrous stock suspension flows from the mixer 34 into a mixing chest37. A storage tank 38 which additionally serves as a filtration unit maybe provided between the mixer 34 and the mixing chest 37. From thestorage tank 38 the filtrate which has been enriched with calciumcarbonate is recycled back into the header tank 7 or into anotherupstream unit for the processing of the dilution water or the fibrousstock suspension.

The mixing chest 37 is equipped with a rotor 39 to thoroughly mix thefibrous stock suspension. The fibrous stock suspension then flows fromthe mixing chest 37 either immediately to a head box in a paper machine,or will be subjected to additional mechanical processing, for example ina refiner feed chest.

Fibrous stock suspension to which calcium hydroxide has not yet beenadded can also be supplied to the mixing chest 37 through a pipe linesystem 1 via the valve 3 and a line 40 in which said valve is installed.

It is further provided that white water or process water which has beenrecovered from the machine for the production of the fibrous stocksuspension, especially from the paper machine, for example in the wirearea of the paper machine or, as already described previously, for thefibrous stock suspension from the dewatering screw 19, is supplied tothe tank 21. Dilution water may for example be supplied to this tankthrough a line 41 which is equipped with a valve 42.

From the container 21 dilution water which is mixed with process waterflows through a line 43, a pump 44 and a valve 45 to the crystallizer25. According to the design of an arrangement for loading of the fibrousstock suspension with a filler, especially with calcium carbonate asdepicted in FIG. 1, a multitude of possibilities arises to influence thecomposition of the fibrous stock suspension that is being produced invarious stages of the manufacturing process.

The installation of a high consistency refiner 46 inside line 4 isadvantageous. A heater 47 for the carbon dioxide which is supplied bythe storage tank 30 is optional. The heater 47 operates with superheatedsteam which is supplied via an inlet 49 and discharged via an outlet 48.

In the production of an SC paper web (SC=super calendered) (FIG. 2) anddepending upon the quality of the paper—SC-A-paper or SC-B paper—certainupper limits for the roughness R for the blackening S (blackeningindex), which is measured in micrometers according to the PPS measuringmethod, must be cited as a function of the roughness. Only papers thathave roughness and blackening values below the limits are usable.

Calendered papers are especially suitable for the rotogravure printingprocess. These papers are calendered in calender nips in order toachieve the desired smoothness of the paper surface, since otherwiseflaws would occur in the print. However, if a calender with aninsufficient number of nips is utilized and if excessive nip pressuresare applied upon the paper web, a portion of the fibers would besqueezed in such a way that they would take on a transparent appearance.In order to achieve a high paper quality this malfunction must notexceed the limits illustrated in FIG. 2 which are determined for variouspaper grades through the limit curves G which are also depicted in FIG.2. Also a too high a waste paper (DIP=deinked paper) component reducesthe paper quality to lower values. Great roughness causes many printingflaws (“missing dots”). However, the production line becomesunprofitable if a multitude of nips are utilized during the printingprocess while at the same time working at a low pressure or low linepressure. In accordance with the current invention the paper quality istherefore improved by loading the cavities of the fibers with ash. Thismakes substantially greater line pressures in the calender nips possibleand the number of calender nips can be reduced compared to the currentstate of the art. This also permits an increase in efficiency of thecalender and at the same time a reduction of the blackening.

A paper machine (FIG. 3) which is suitable for the production ofcalendered papers comprises a twin wire former 50, a press section 51with two pairs of rolls arranged in tandem, wherein in each instance onepress roll is in the embodiment of a shoe press roll.

A single row dryer section 52 is located adjacent to the press section51. The paper web is calendered in a calender 53 which is equipped witha multitude of roll pairs. The paper web is subsequently wound in awinder 54.

While this invention has been described with respect to at least oneembodiment, the present invention can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

1. A method for producing a calendered paper web in a paper machine,said method comprising the step of partially loading with ash aplurality of fibers of a fibrous stock suspension.
 2. The method inaccordance with claim 1, further comprising running the paper webthrough a calender which is equipped with a plurality of nips.
 3. Themethod in accordance with claim 2, further comprising applying aplurality of line loads greater than 50 kN/m to the paper web in saidplurality of nips.
 4. The method in accordance with claim 2, furthercomprising applying a plurality of line loads greater than 100 kN/m tothe paper web.
 5. The method in accordance with claim 2, furthercomprising applying a plurality of line loads greater than 350 kN/m tothe paper web.
 6. The method in accordance with claim 2, furthercomprising drying the paper web to a moisture content of less than 5%and subsequently remoistening the paper web to a moisture content ofmore than 7% prior to said calender.
 7. A paper machine for theproduction of a calendered paper web from a fibrous stock suspensionincluding a plurality of fibers which are partially loaded with ash,said paper machine comprising a first device for loading a fibrous stocksuspension with ash.
 8. The paper machine in accordance with claim 7,wherein said first device comprises a first static mixer, a preparationunit for adding one of calcium oxide and calcium hydroxide, one of apress and a dewatering screw, one of an equalizing reactor and anequalizing screw, a container serving as a crystallizer, a second staticmixer, and one of a carbon dioxide storage tank and a second device forrecovering carbon dioxide.
 9. The paper machine in accordance with claim8, further comprising at least one of a High Consistency Cleaner, acarbon dioxide heater, and a storage tank one of for a press water andfor a water recovered in said dewatering screw.
 10. The paper machine inaccordance with claim 8, further comprising a line and one of a headertank and an upstream third device, said line configured for returningany filtrate recovered in said dewatering screw one of to said headertank and to said upstream third device for preparing the fibrous stocksuspension.
 11. The paper machine in accordance with claim 8, furthercomprising a washer apparatus for cleaning the fibrous stock suspension,said washer apparatus located after said container serving as saidcrystallizer.
 12. The paper machine in accordance with claim 7, furthercomprising a twin wire former.
 13. The paper machine in accordance withclaim 7, further comprising one press section with at least one shoepress.
 14. The paper machine in accordance with claim 7, furthercomprising a calender with a plurality of nips.
 15. The paper machine inaccordance with claim 14, further comprising an arrangement forremoistening of the paper web, said arrangement located before saidcalender.
 16. The paper machine in accordance with claim 15, whereinsaid arrangement comprises a steam blow box.
 17. The paper machine inaccordance with claim 7, further comprising a coating unit.